Round bone sirloin. The Grocer’s Encyclopedia (1911). rawpixel.com / flickr / CC BY 4.0
A round bone sirloin steak is cut from the sirloin area of the cow. In the steak there is a cross-section of a round bone.The steak is a cross-section of meat muscles including the bottom round (aka outside round), the tri-tip, the top sirloin and the tenderloin.
The steaks cut in the sirloin are, in order from more tender to less tender: pin bone, flat bone, round bone and wedge bone.
If the bone is removed, the steak will be sold as a top sirloin steak.
This is different from a round bone (aka arm) steak, which is from the chuck area.